Before the advent of PHP-GTK, PHP could only be used to develop Web applications. In other words, PHP required a Web server such as Apache or IIS to function and it could not run on its own. This meant that everything PHP did was server-side and the closest you could come to writing stand-alone applications would be to run a server on your computer.
GTK+, which stands for GIMP Tool Kit (GIMP being the GNU Image Manipulation Program), is an open source application that works on multiple platforms. The GTK home page, www.gtk.org (Figure 12.16), describes the system's origins and usage but from a C programming perspective (it, like PHP, is written in C).